1. Field of the Invention
The invention relates to a method of operating a burner.
2. Discussion of Background
So-called solid-jet atomizers, inter alia, are used in order to atomize liquid fuels. In such a nozzle, the liquid fuel is ejected under high pressure from a prechamber through a circular injection orifice of a certain guide length. The resulting fuel jet disintegrates in a more or less static environment to form a fine spray. In order to produce a sufficiently fine spray for the combustion, however, a relatively high fuel pressure is required, as is applied only during full load of a gas-turbine plant. On the other hand, on account of the low fuel flow rate, only a low fuel pressure is required, for example, during the ignition of a combustion chamber or when running the plant up to speed after the ignition. However, since the atomization of the liquid fuel by means of a solid-jet atomizer naturally leads to relatively large droplets during part load of the gas-turbine plant, the conventional solid-jet atomizers are not suitable for the part-load operation of a gas-turbine plant.
In order to nonetheless obtain fine droplets, the nozzle may additionally be provided with a so-called ignition stage. This ignition stage is a second atomizer which is designed for correspondingly low flow rates and therefore ensures sufficiently fine atomization of the liquid fuel during part load. Such a solution is disclosed by the textbook "Atomization and sprays", by A. Lefebvre, West Lafayette, Ind. 1989, page 120, FIG. 4.21. However, this nozzle, with its two fuel feed lines and with two fuel ducts which lie radially one above the other and to which fuel is admitted according to the fuel mass flow required, requires a relatively large construction space. In addition, the components used are naturally of intricate design, as a result of which the nozzle is more susceptible to trouble. The use of more than two atomizers in one nozzle adds to said disadvantages. In addition, a corresponding number of fuel feed lines with various control valves are necessary, as a result of which not only the design input but also the costs increase. During changeover to the atomizer required in each case, discontinuity occurs in the fuel flow and this may lead to extinction of the burner.
Also disclosed by the textbook "Atomization and sprays", by A. Lefebvre, West Lafayette, Indiana 1989, on pages 142-144 and FIG. 4.50, is an atomizer. for liquid fuels, in which atomizer an auxiliary gas is introduced into the liquid flow upstream of the injection orifice. To this end, a gas tube is arranged in the interior of the liquid-fuel tube, which gas tube ends upstream of the injection orifice and is provided with a plurality of discharge orifices for the auxiliary gas. The auxiliary gas is injected into the liquid flow at a low velocity and at a pressure only marginally higher than that of the liquid flow. The auxiliary gas issuing into the liquid forms gas bubbles, the effect of which is to produce relatively thin shreds and ribbons of liquid in the liquid flow. Since such liquid flows of smaller diameter are easier to break up into a fine spray, the atomization of the liquid fuel is improved in this way. The total volumetric flow to be atomized is increased by the injection of the auxiliary gas into the liquid-fuel tube, so that sufficient atomization of the fuel can be achieved by means of a solid-jet atomizer even during part load.
The disadvantage of such an atomizer, however, is that it cannot be used during full load of the gas-turbine plant, i.e. at high fuel pressure. In order to ensure the injection of the auxiliary gas into the liquid fuel and thus the operability of the atomizer during this operating state, the auxiliary gas must be highly compressed. However, this is very expensive and is not possible without an external supply of energy. Therefore such atomizers have not been widely adopted hitherto.